Snow Melting - Idle/On Controls

A more advanced control would maintain a minimum standby surface temperature at, or just below, freezing and then move into the on mode in the presence of snow. In this manner the time needed to bring the surface temperature above freezing is significantly shortened allowing the heat to move into the snow faster, turning it into liquid, and then evaporating it and/or draining it away to maintain a clean surface. The  same higher end controls can be programmed to run for a certain amount of time following snowstorms to ensure that any residual accumulation is cleared. However, in a mild year with very little snowfall this strategy could be more expensive to run then the on/off method. This comparison illustrates that storm characteristics play a major role in how control strategies can pass or fail to meet a client’s needs and influence operating costs. In reality, what a client experiences over the life expectancy of the system in terms of performance and efficiency, is determined by the size of their initial investment in the system and controls.

Click here to see a snow meting piping and control schematic.

Performance

Let’s look at the engineering of systems to determine what a control system must manage when asked to perform. Those who have studied thermal dynamics know that energy cannot be saved although it makes for an interesting discussion with those that use the word ‘save’ in marketing initiatives. Energy cannot be created nor can it be destroyed therefore it cannot be saved. It is just moved around from one place (form) to another. What a client pays in exchange for converting energy from one form to another and the benefits received in the process are defined by the efficiency of the conversion. In the case of a fuel-fired boiler used to heat snow melt fluid, energy in the gas or oil is released to the liquid through the combustion heat exchange process. Energy not dispersed into the fluid is radiated, conducted, evaporated or convected into the boiler room or up the chimney. The amount of heat required in the antifreeze/water and the electrical power to move it is a function of several factors including, but not limited to, tube spacing, depth, soil conditions, storm characteristics, slab dimension, and so on. Control systems actually regulate the flow of energy. We define the energy required at the surface as heat flux.
Image credit: Development of a Two Dimensional Transient Model of Snow-Melting Systems, and Use of the Model for Analysis of Design Alternatives, ASHRAE 1090-RP, 2001, Spitler, Rees, Xia, Chulliparambil, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. and Oklahoma State University.

The steady state heat flux calculation for melting snow is:

qo = qs + qm + Ar( qh+ qe), where

        qo = heat flux required at snow- melting surface, Btuh x ft²
        qs = sensible heat flux, Btuh x ft²
        qm= latent heat flux, Btuh x ft²
        Ar = snow- free area ratio, dimensionless
        qh = convective and radiative heat flux from snow- free surface,
                Btuh x ft²
        qe = heat flux of evaporation, Btuh x ft²
 

Click here for part I, Introduction
Click here for part II, Manual Controls
Click here for part IV, Area Free Ratio and Frequency Percentile  
Click here for part V, Conclusion        

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